This invention relates to evaporators, and more particularly to an evaporator with increased heat transfer between the vapors being condensed and the liquid being evaporated. Such evaporators may be used, for example, to concentrate pulp and paper spent black liquor, and distillery or brewery solubles, to desalinate sea water, or to process chemicals such as ammonium nitrate. Evaporators may have either single or multiple effects.
Shop fabrication of evaporator containers is more economical than field erection, but the diameter of such bodies is limited by legal and physical restrictions on the diameter of objects which can be transported by a carrier vehicle. Therefore, evaporators are being constructed with relatively long tubes in order to maximize the heat transfer surface that can be packaged in a transportable shop-built container. Vertical tube evaporators have been made with tube lengths that range up to thirty-two feet. We have observed that when such tube lengths exceed about twenty feet, the increase in heat transferred is proportionately less than the increase in heat transfer surface. Also, there are legal and physical restrictions on the length of objects which can be transported by vehicle, so this limits the amount of evaporator capacity that can be added by increasing tube length.